Non-oxide ceramic matrix composites for application in hot gas atmospheres – requirements and potential

In spite of the ambitious efforts to increase the portion of alternative and renewable resources the energy production based on fossil fuels will still represent the main part of energy in the next years. Caused by the increasing energy price and the stronger requirements in environmental protection the main focus of future generations of gas turbines will be emphasized on an increased efficiency with a simultaneous reduction of the emissions. From technical point of view these goals can be obtained only by higher hot gas temperatures. Ceramic matrix composites (CMC) offer a high potential for applications as structural parts in advanced gas turbines. During recent years, significant progress in material development of oxide and non-oxide CMC has been achieved, however, there are still considerable deficits especially in the long-term behavior of the materials in hot gas conditions. The present study is focused on the environmental stability of the materials. Caused by the high water vapor pressure in combination with high temperatures and gas velocities, corrosion processes at the surface and inside the materials were observed resulting in significant material degradation and mass loss. Hence, environmental barrier coatings (EBC) have been presented to be the solution to protect the surface of the ceramic materials. Systematic studies on the hot gas corrosion of non-oxide CMC have been performed with and without EBC. Based on a detailed understanding of the processes in the whole system, EBC and the ceramic base material during application in hot gas environments at elevated temperatures, general concepts for the development of environmental barrier coatings will be discussed

2 MW peak power generation in fluorine co-doped Yb fiber prepared by powder-sinter technology

We report on the first, to the best of our knowledge, implementation of a fluorine co-doped large-mode-area REPUSIL fiber for high peak power amplification in an ultrashort-pulse master oscillator power amplifier. The core material of the investigated step-index fiber with high Yb-doping level, 52 µm core and high core-to-clad ratio of 1:4.2 was fabricated by means of the REPUSIL powder-sinter technology. The core numerical aperture was adjusted by fluorine codoping to 0.088. For achieving high beam quality and for ensuring a monolithic seed path, the LMA fiber is locally tapered. We demonstrate an Yb fiber amplifier with near-diffraction-limited beam quality of M2 = 1.3, which remains constant up to a peak power of 2 MW. This is a record for a tapered single core fiber. © 2020 Optical Society of Americ

Verfahren zur Herstellung dichter Bauteile aus Zirkoncarbid und mit dem Verfahren hergestellte Bauteile

Die Erfindung betrifft ein Verfahren zur Herstellung gesinterter Bauteile aus Zirkoncarbid sowie mit dem Verfahren hergestellte Bauteile. Zur Herstellung der Bauteile wird ein Grünköper mit einem unterstöchiometrischen ZrCxOy Pulver, bei dem X = 0,7 bis 1,0 und Y = 0,0 bis 0,2 hergestellt und anschließend wird der Grünkörper bei einer Wärmebehandlung mit einer Maximaltemperatur von 2000 °C drucklos gesintert

Pressureless sintering of ZrC with variable stoichiometry

This paper presents the experiments on the synthesis of zirconium carbide (ZrC) using carbothermal reduction of zirconia (ZrO2). The ratio of ZrO2:C is used to adapt ZrCxOy with x < 1 or ZrC + C. The modification of ZrCxOy and the total carbon amount allows the use of pressureless sintering method in combination with sintering temperatures ≤ 2000 °C. Fully densified ZrC products are obtained. The relevant details of ZrC formation are investigated by X-ray diffraction (XRD). The sintered products are characterized by XRD, field emission scanning electron microscopy (FESEM), as well as mechanical and electrical methods. XRD and FESEM investigations show that ZrCxOy is formed during the manufacturing process. The grain size and additional zirconia or carbon are related to the ZrO2:C ratio of the starting powder mixture. Bending strength up to 300 MPa, Young’s modulus up to 400 GPa, fracture toughness up to 4.1 MPa·m1/2, and electrical resistance at room temperature around 10−4 Ω·cm are reached by the pressureless sintered ZrC

Lokale Beeinflussung des Gussgefüges durch den gezielten Einsatz von Formstoffen

In einem öffentlich geförderten Projekt (SAB 100222766) arbeiteten DirektForm und Fraunhofer IKTS gemeinsam an neuen Konzepten für Formstoffe, die gezielt zur Einstellung des sich ausbildenden Werkstoffgefüges von Gusswerkstoffen eingesetzt werden können. Dazu wurden bekannte Formstoffe wie Quarzsand, Chromerzsand und Siliciumcarbid als Ausgangsstoffe verwendet. Als entscheidende Parameter für die resultierende Wärmeableitung in den Formstoffen zeigte sich neben der Korngrößenverteilung der eingesetzte Binder. Deshalb wurde ein Ballclay Bindermaterial eingesetzt, der sich deutlich von den üblichen Harzbindern abhebt. Zusätzlich ist es mit einer auf den Binder abgestimmten Herstellungsroute möglich, den Formstoff kostengünstig zu bearbeiten und somit für die Prototypen oder Kleinserienfertigung optimale Formen herzustellen. In dem Beitrag werden die experimentellen Ergebnisse zur Binder- und Formstoffauswahl, die Herstellung und Bearbeitung der neuen Formstoffe sowie die untersuchten Wechselwirkungen zwischen Formstoff und Werkstoffgefüge des Gussmaterials beschrieben. Für eine praktische Umsetzung stehen noch weiterführende und vertiefende Arbeiten an

Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers

The production of carbon fibers from polyacrylonitrile (PAN) includes a stabilization step before carbonization. Transferring this stabilization from the laboratory to a bigger scale cannot be performed without changes of the process parameters. Using a pure PAN polymer, fibers were spun and their stabilization as well as carbonization was studied in laboratory scale. The resulting materials were analyzed with FTIR, SEM and XRD. The stabilization step was then transferred to a pilot scale. Limitations given by the production line had to be taken into account to develop a suitable temperature treatment. The formation of a core/shell structure during stabilization was observed and was detrimental to the properties of the fiber after carbonization. Adjustments were made based on obtained results to balance the negative effects of high heating rates and strong temperature gradients. With the implemented changes the mechanical properties could be reproduced on the larger production scale